SPARC induces the expression of type 1 plasminogen activator inhibitor in cultured bovine aortic endothelial cells

Secreted Protein Acidic and Rich in Cysteine in Ocular Tissue

Secreted protein acidic and rich in cysteine (SPARC), also known as osteonectin or BM-40, is the prototypical matricellular protein. Matricellular proteins are nonstructural secreted proteins that provide an integration between cells and their surrounding extracellular matrix (ECM). Regulation of the ECM is important in maintaining the physiologic function of tissues. Elevated levels of SPARC have been identified in a variety of diseases involving pathologic tissue remodeling, such as hepatic fibrosis, systemic sclerosis, and certain carcinomas. Within the eye, SPARC has been identified in the trabecular meshwork, lens, and retina. Studies have begun to show the role of SPARC in these tissues and its possible role, specifically in primary open-angle glaucoma, cataracts, and proliferative vitreoretinopathy. SPARC may, therefore, be a therapeutic target in the treatment of certain ocular diseases. Further investigation into the mechanism of action of SPARC will be necessary in the development of SPARC-targeted therapy.

Molecular cloning and functional analysis of an orange-spotted grouper (Epinephelus coioides) secreted protein acidic and rich in cysteine (SPARC) and characterization of its expression response to nodavirus

Mammalian secreted protein acidic and rich in cysteine (SPARC) is the primary regulator of cell shape and cell adhesion to fibronectin. We, for the first time, report the complete sequencing of SPARC cDNA from orange-spotted grouper. Despite the difference in the lengths of the SPARC transcripts, all of the SPARC molecules encoded a signal peptide, follistain-like copper binding sequence (KGHK) domain, and extracellular domain. The grouper SPARC gene was differentially expressed in vivo and contributed differently to high-level expression of SPARC in muscle. Immunohistochemical staining demonstrated a decreased level of SPARC in nodavirus-infected grouper compared with healthy grouper. Comparative real-time polymerase chain reaction analyses of eye tissues of viral nervous necrosis grouper and healthy grouper were performed. Recombinant SPARC produced changes in grouper cell shape 24 h after treatment. The results provide new insight into the pathogenesis of nodavirus, and demonstrate an experimental rationale for SPARC characterization in nodavirus-infected grouper.

SPARC suppresses apoptosis of idiopathic pulmonary fibrosis fibroblasts through constitutive activation of beta-catenin

Idiopathic pulmonary fibrosis (IPF) is a poorly understood progressive disease characterized by the accumulation of scar tissue in the lung interstitium. A hallmark of the disease is areas of injury to type II alveolar epithelial cells with attendant accumulation of fibroblasts in areas called fibroblastic foci. In an effort to better characterize the lung fibroblast phenotype in IPF patients, we isolated fibroblasts from patients with IPF and looked for activation of signaling proteins, which could help explain the exaggerated fibrogenic response in IPF. We found that IPF fibroblasts constitutively expressed increased basal levels of SPARC, plasminogen activator inhibitor-1 (PAI-1), and active beta-catenin compared with control cells. Control of basal PAI-1 expression in IPF fibroblasts was regulated by SPARC-mediated activation of Akt, leading to inhibition of glycogen synthase kinase-3beta and activation of beta-catenin. Additionally, IPF fibroblasts (but not control fibroblasts) were resistant to plasminogen-induced apoptosis and were sensitized to plasminogen-mediated apoptosis by inhibition of SPARC or beta-catenin. These findings uncover a newly discovered regulatory pathway in IPF fibroblasts that is characterized by elevated SPARC, giving rise to activated beta-catenin, which regulates expression of downstream genes, such as PAI-1, and confers an apoptosis-resistant phenotype. Disruption of this pathway may represent a novel therapeutic target in IPF.

Spaceflight modulates expression of extracellular matrix, adhesion, and profibrotic molecules in mouse lung

NASA has reported pulmonary abnormalities in astronauts on space missions, but the molecular changes in lung tissue remain unknown. The goal of the present study was to explore the effects of spaceflight on expression of extracellular matrix (ECM), cell adhesion, and pro-fibrotic molecules in lungs of mice flown on Space Shuttle Endeavour (STS-118). C57BL/6Ntac mice housed in animal enclosure modules during a 13-day mission in space (FLT) were killed within hours after return; ground controls were treated similarly for comparison (GRD). Analysis of genes associated with ECM and adhesion molecules was performed according to quantitative RT-PCR. The data revealed that FLT lung samples had statistically significant transcriptional changes, i.e., at least 1.5-fold, in 25 out of 84 examined genes (P < 0.05); 15 genes were upregulated and 10 were downregulated. The genes that were upregulated by more than twofold were Ctgf, Mmp2, Ncam1, Sparc, Spock1, and Timp3, whereas the most downregulated genes were Lama1, Mmp3, Mmp7, vcam-1, and Sele. Histology showed profibrosis-like changes occurred in FLT mice, more abundant collagen accumulation around blood vessels, and thicker walls compared with lung samples from GRD mice. Immunohistochemistry was used to compare expression of six selected proteins associated with fibrosis. Immunoreactivity of four proteins (MMP-2, CTGF, TGF-beta1, and NCAM) was enhanced by spaceflight, whereas, no difference was detected in expression of MMP-7 and MMP-9 proteins between the FLT and GRD groups. Taken together, the data demonstrate that significant changes can be readily detected shortly after return from spaceflight in the expression of factors that can adversely influence lung function.

Circulating stromal osteonectin-positive progenitor cells and stenotic coronary atherosclerosis

The level of circulating stromal progenitor cells carrying osteonectin (ON), a marker of osteogenic differentiation, was evaluated by flow cytometry in blood of patients with coronary artery disease (CAD). Ninety-nine patients with CAD were included into the study. Coronary angiography of all patients showed critical stenosis of at least 2 coronary arteries or their major branches. The control groups included 8 patients without CAD and 19 healthy volunteers. In control patients, no lesions of the coronary bed were found by angiography. The absence of CAD in the volunteers was confirmed by bicycle stress test. The content of ON-positive cells in blood was examined in various populations of lymphocyte-like cells. It was found that the number of ON+ lymphocyte-like cells with CD41 positivity in blood of patients without coronary stenosis (0.27%+/-0.11%, mean+/-SD) did not differ significantly from corresponding value in healthy volunteers (0.26%+/-0.07%, p=0.94). In CAD patients, the percent of these ON+ cells was 1.01%+/-0.49% and was significantly higher than in blood of healthy volunteers (p<0.0001) and patients without CAD (p<0.0001). High content of ON+ lymphocyte-like cells with CD41 positivity in blood may serve as noninvasive marker of arterial atherosclerosis.

Accelerated dendritic-cell migration and T-cell priming in SPARC-deficient mice

On their path to draining lymph nodes, epidermal Langerhans cells traverse collagen-dense connective tissue before reaching lymphatic vessels. The matricellular protein SPARC (secreted protein, acidic and rich in cysteine), which is induced during inflammation and tissue repair, organizes collagen deposition in tissue stroma. We analyzed Langerhans cell and dendritic-cell migration and its impact on T-cell priming in SPARC-null (SPARC–/–) and SPARC-sufficient (SPARC+/+) mice. Although the same number of Langerhans cells populate the ear skin of SPARC–/– and SPARC+/+ mice, more Langerhans cells were found in the lymph nodes draining antigen-sensitized ears of SPARC–/– mice and significantly more Langerhans cells migrated from null-mice-derived ear skin explants. Such favored Langerhans cell migration is due to the host environment, as demonstrated by SPARC+/+&gt;SPARC–/– and reciprocal chimeras, and have a profound influence on T-cell priming. Contact-, delayed type-hypersensitivity and naive T-cell receptor-transgenic T-cell priming, together indicate that the lack of SPARC in the environment accelerates the onset of T-cell priming by hastening Langerhans cell/dendritic-cell migration.

Differential effect of Cyclosporin A and FK506 on SPARC mRNA expression by human gingival fibroblasts

BACKGROUND

Secreted protein acidic and rich in cysteine (SPARC) is a glycoprotein that mediates cell-matrix interactions. In adults, its expression is mostly limited to tissue undergoing remodeling. During the development of Cyclosporin A (CsA)-induced gingival overgrowth (GO) a remodeling of the connective compartment occurs. By contrast, clinical trials showed that FK506 is not related to GO. SPARC expression and its involvement in GO is unknown. Our aim was, therefore, to analyze the effect of CsA and FK506 on SPARC gene expression.

METHODS

Cultured human gingival fibroblasts were incubated with CsA, FK506 or with their vehicle (VH) for 24, 48 and 72 h. SPARC gene expression was determined by RT-PCR.

RESULTS

SPARC mRNA levels tended to increase 72 h after CsA treatment, whilst they are undetectable in FK506-treated fibroblasts, compared to VH.

CONCLUSION

This gene expression profile is consistent with the involvement of SPARC in the mechanisms leading to the development of CsA-induced GO. By contrast, the undetectable SPARC mRNA levels in FK506-treated fibroblasts suggest that FK506 may be associated with a role of ECM stabilization, that does not induce GO.

Development of UV-induced squamous cell carcinomas is suppressed in the absence of SPARC

SPARC (Secreted Protein Acidic and Rich in Cysteine) is a multifunctional glycoprotein belonging to a group of matrix-associated factors that mediate cell-extracellular matrix interactions but have no structural roles. In the present study we investigated the contribution of SPARC to factors that influence the development of skin tumors in response to UV irradiation. A hairless SPARC-null mouse was developed and compared to control SKH1 hairless mice in terms of skin tumor induction and extracellular matrix changes occurring in response to UV-irradiation. Following 23 weeks of exposure to UVB totaling 14.5 J per cm(2), tumor development in the wild-type mice was severe, with an average of over 20 tumors per mouse, many of which were squamous cell carcinomas. Conversely, the SPARC-null mice were strikingly tumor-resistant, developing no squamous cell carcinomas and averaging less than one small papilloma per mouse. SPARC was undetectable immunohistochemically in skin from the non-irradiated control group yet was present in relatively high quantities in the basal and superficial areas of the tumor mass. The SPARC-null mice also exhibited a limited contact hypersensitivity response and were refractory to UV induced immune suppression. In conclusion, SPARC appears to have a crucial role in mediating tumor formation in response to UV irradiation.

SPARC inhibits epithelial cell proliferation in part through stimulation of the transforming growth factor-beta-signaling system

Secreted protein, acidic and rich in cysteine (SPARC) is a multifunctional secreted protein that regulates cell-cell and cell-matrix interactions, leading to alterations in cell adhesion, motility, and proliferation. Although SPARC is expressed in epithelial cells, its ability to regulate epithelial cell growth remains largely unknown. We show herein that SPARC strongly inhibited DNA synthesis in transforming growth factor (TGF)-beta-sensitive Mv1Lu cells, whereas moderately inhibiting that in TGF-beta-insensitive Mv1Lu cells (i.e., R1B cells). Overexpression of dominant-negative Smad3 in Mv1Lu cells, which abrogated growth arrest by TGF-beta, also attenuated growth arrest stimulated by SPARC. Moreover, the extracellular calcium-binding domain of SPARC (i.e., SPARC-EC) was sufficient to inhibit Mv1Lu cell proliferation but not that of R1B cells. Similar to TGF-beta and thrombospondin-1, treatment of Mv1Lu cells with SPARC or SPARC-EC stimulated Smad2 phosphorylation and Smad2/3 nuclear translocation: the latter response to all agonists was abrogated in R1B cells or by pretreatment of Mv1Lu cells with neutralizing TGF-beta antibodies. SPARC also stimulated Smad2 phosphorylation in MB114 endothelial cells but had no effect on bone morphogenetic protein-regulated Smad1 phosphorylation in either Mv1Lu or MB114 cells. Finally, SPARC and SPARC-EC stimulated TGF-beta-responsive reporter gene expression through a TGF-beta receptor- and Smad2/3-dependent pathway in Mv1Lu cells. Collectively, our findings identify a novel mechanism whereby SPARC inhibits epithelial cell proliferation by selectively commandeering the TGF-beta signaling system, doing so through coupling of SPARC-EC to a TGF-beta receptor- and Smad2/3-dependent pathway.

Quantitative analysis of osteonectin mRNA in thyroid carcinomas

Our recent study of the gene expression profile in thyroid carcinoma showed an overexpression of osteonectin mRNA, an extracellular matrix protein, in an anaplastic carcinoma. To confirm this, we measured the expression levels of osteonectin mRNA in 84 thyroid normal and tumor tissues, including five anaplastic carcinomas by realtime quantitative reverse-transcription PCR. Increased expression of osteonectin mRNA was observed in anaplastic carcinoma tissue. However, in five anaplastic carcinoma cell lines, no increase was observed in the expression levels of osteonectin mRNA. These findings suggest the possibility that increased expression of osteonectin mRNA in anaplastic carcinoma tissue may be due to its overexpression in stromal cells, but not in anaplastic carcinoma cells.

SPARC expression in primary human renal cell carcinoma: upregulation of SPARC in sarcomatoid renal carcinoma

SPARC (secreted protein acidic and rich in cysteine, also called osteonectin, BM-40, and 43K protein) is a matricellular protein and is associated with cell-matrix interactions during cell proliferation and extracellular remodeling. It is also implicated in the neovascularization, invasion, and metastasis of human malignancies. To investigate a potential role of the SPARC in renal tumorigenesis, we examined primary renal cell carcinomas (RCCs) for SPARC expression by Northern blot analysis and for protein distribution by immunohistochemistry. We found that 6 (100%) of 6 sarcomatoid and 25 (70%) of 36 clear-cell carcinomas had enhanced SPARC transcription compared with that of the corresponding normal kidney tissue. In contrast, papillary and chromophobe RCCs characterized by a hypovascular or avascular tumor phenotype had undetectable SPARC expression. Immunohistochemical analysis showed that SPARC was strongly stained in the cytoplasm of the sarcomatoid neoplastic cells in sarcomatoid RCCs, whereas it was expressed only in the vascular endothelial cells and fibroblasts in clear-cell RCCs. SPARC staining intensity in the stromal cells was increased in the invading portion in some clear-cell RCCs. These findings suggest that tumor development, including neovascularization and invasion in clear-cell RCCs, might be regulated by SPARC from stromal endothelial cells and fibroblasts and that sarcomatoid transformation from common-type RCCs is associated with upregulation of SPARC expression; SPARC may contribute to its aggressive tumor phenotype.

Induction of SPARC by VEGF in human vascular endothelial cells

SPARC/osteonectin/BM-40 is a matricellular protein that is thought to be involved in angiogenesis and endothelial barrier function. Previously, we have detected high levels of SPARC expression in endothelial cells (ECs) adjacent to carcinomas of kidney and tongue. Although SPARC-derived peptide showed an angiogenic effect, intact SPARC itself inhibited the mitogenic activity of vascular endothelial growth factor (VEGF) for ECs by the inhibiting phosphorylation of flt-1 (VEGF receptor 1) and subsequent ERK activation. Thus, the role of SPARC in tumor angiogenesis, stimulation or inhibition, is still unclear. To clarify the role of SPARC in tumor growth and progression, we determined the effect of VEGF on the expression of SPARC in human microvascular EC line, HMEC-1, and human umbilical vein ECs. VEGF increased the levels of SPARC protein and steady-state levels of SPARC mRNA in serum-starved HMEC-1 cells. Inhibitors (SB202190 and SB203580) of p38, a mitogen-activated protein (MAP) kinase, attenuated VEGF-stimulated SPARC production in ECs. Since intact SPARC inhibits phosphorylation ERK MAP kinase in VEGF signaling, it was suggested that SPARC plays a dual role in the VEGF functions, tumor angiogenesis, and extravasation of tumors mediated by the increased permeability of endothelial barrier function.

The expression of SPARC in adipose tissue and its increased plasma concentration in patients with coronary artery disease

OBJECTIVE

Adipocytes secrete various cytokines and matrix proteins. Several of them precipitate in obesity-associated diseases, including atherosclerosis. In the current study, we have examined the expression of secreted protein, acidic and rich in cysteine (SPARC) in adipose tissue and its significance in obesity and coronary artery disease (CAD).

RESEARCH METHODS AND PROCEDURES

The SPARC mRNA expressions both in vivo and in vitro were detected by Northern blot analysis. Plasma SPARC concentrations were measured by enzyme immunosorbent assay. First, we investigated the plasma SPARC levels of 88 unrelated adult Japanese subjects (62 men and 26 women; average age: [+/- SD] 50 +/- 12 years; body mass index [BMI]: 16 to 46 kg/m(2)). Additionally 31 subjects with CAD diagnosed by coronary angiography (20 men and 11 women) were also investigated.

RESULTS

Human adipose tissues expressed abundant SPARC mRNA. SPARC expression in adipose tissues was upregulated in obese db/db mice. Markedly enhanced expression of SPARC mRNA was observed in 3T3-L1 fibroblasts during adipocyte differentiation. Consistent with these results, plasma SPARC levels proved a positive correlation with BMI in humans (r = 0.27; p < 0.01). Interestingly, plasma SPARC concentrations were significantly elevated in age- and BMI-matched subjects with CAD (p < 0.05).

DISCUSSION

SPARC was expressed in adipose tissues and its expression was enhanced in obese mice. In human, plasma SPARC levels were elevated in obesity and CAD patients. This elevated SPARC may be involved in the progression of CAD.

The matricellular protein SPARC/osteonectin as a newly identified factor up-regulated in obesity

Alterations in the expression level of genes may contribute to the development and pathophysiology of obesity. To find genes differentially expressed in adipose tissue during obesity, we performed suppression subtractive hybridization on epididymal fat mRNA from goldthioglucose (GTG) obese mice and from their lean littermates. We identified the secreted protein acidic and rich in cysteine (SPARC), a protein that mediates cell-matrix interactions and plays a role in modulation of cell adhesion, differentiation, and angiogenesis. SPARC mRNA expression in adipose tissue was markedly increased (between 3- and 6-fold) in three different models of obesity, i.e. GTG mice, ob/ob mice, and AKR mice, after 6 weeks of a high fat diet. Immunoblotting of adipocyte extracts revealed a similar increase in protein level. Using a SPARC-specific ELISA, we demonstrated that SPARC is secreted by isolated adipocytes. We found that insulin administration to mice increased SPARC mRNA in the adipose tissue. Food deprivation had no effect on SPARC expression, but after high fat refeeding SPARC mRNA levels were significantly increased. Our results reveal both hormonal and nutritional regulation of SPARC expression in the adipocyte, and importantly, its alteration in obesity. Finally, we show that purified SPARC increased mRNA levels of plasminogen activator inhibitor 1 (PAI-1) in cultured rat adipose tissue suggesting that elevated adipocyte expression of SPARC might contribute to the abnormal expression of PAI-1 observed in obesity. We propose that SPARC is a newly identified autocrine/paracrine factor that could affect key functions in adipose tissue physiology and pathology.

Regional and cellular localization of osteonectin/SPARC expression in connective tissue and cytotrophoblastic layers of human fetal membranes at term

Fetal membranes overlying the cervix in patients prior to and during labour, and within the rupture tear after spontaneous delivery at term, exhibit altered morphology. In this study we report that in comparison to mid-zone fetal membranes biopsies, these regions are characterized by increased expression of the matricellular protein osteonectin or SPARC (Secreted Protein Acidic and Rich in Cysteine). In the reticular layer, the percentage of vimentin positive mesenchymal cells immunoreactive for osteonectin increased in these regions from 3-4% to 25-33% and represented a fraction of the alpha-smooth muscle actin positive myofibroblasts elevated in the same regions. In the fibroblastic layer, the percentage of osteonectin positive cells increased from 1-5% to 8-13%; however, these did not exhibit the same relationship to the alpha-smooth muscle actin positive myofibroblasts in this layer. In the cytotrophoblastic layer the percentage of cytotrophoblastic cells immunoreactive for osteonectin increased from 1% to 6-12%. Elevation of in-situ detectable mRNA was also observed in the same cellular populations in this region. The incidence of cells positive for osteonectin mRNA or protein in the reticular layer correlated with morphological changes. Osteonectin has been implicated in the regulation of extracellular matrix turnover, and its pattern of expression suggests a role in the regional connective tissue and cytotrophoblastic changes proposed to be involved in the cleavage and rupture of fetal membranes.

SPARC, a matricellular glycoprotein with important biological functions

SPARC (secreted protein, acidic and rich in cysteine) is a unique matricellular glycoprotein that is expressed by many different types of cells and is associated with development, remodeling, cell turnover, and tissue repair. Its principal functions in vitro are counteradhesion and antiproliferation, which proceed via different signaling pathways. SPARC consists of three domains, each of which has independent activity and unique properties. The extracellular calcium binding module and the follistatin-like module have been recently crystallized. Specific interactions between SPARC and growth factors, extracellular matrix proteins, and cell surface proteins contribute to the diverse activities described for SPARC in vivo and in vitro. The location of SPARC in the nuclear matrix of certain proliferating cells, but only in the cytosol of postmitotic neurons, indicates potential functions of SPARC as a nuclear protein, which might be involved in the regulation of cell cycle progression and mitosis. High levels of SPARC have been found in adult eye, and SPARC-null mice exhibit cataracts at 1-2 months of age. This animal model provides an excellent opportunity to confirm and explore some of the properties of SPARC, to investigate cataractogenesis, and to study SPARC-related family proteins, e.g., SC1/hevin, a counteradhesive matricellular protein that might functionally compensate for SPARC in certain tissues.(J Histochem Cytochem 47:1495-1505, 1999)

Collagen accumulation is decreased in SPARC-null mice with bleomycin-induced pulmonary fibrosis

Secreted protein acidic and rich in cysteine (SPARC) has been shown to be coexpressed with type I collagen in tissues undergoing remodeling and wound repair. We speculated that SPARC is required for the accumulation of collagen in lung injury and that its absence would attenuate collagen accumulation. Accordingly, we have assessed levels of collagen in SPARC-null mice in an intratracheal bleomycin-injury model of pulmonary fibrosis. Eight- to ten-week-old SPARC-null and wild-type (WT) mice received bleomycin (0.0035 U/g) or saline intratracheally and were subsequently killed after 14 days. Relative levels of SPARC mRNA were increased 2.7-fold (P < 0.001) in bleomycin-treated WT lungs in comparison with saline-treated lungs. Protein from bleomycin-treated WT lung contained significantly more hydroxyproline (191.9 microg/lung) than protein from either bleomycin-treated SPARC-null lungs or saline-treated WT and SPARC-null lungs (147.4 microg/lung, 125.4 microg/lung, and 113. 0 microg/lung, respectively; P < 0.03). These results indicate that SPARC is increased in response to lung injury and that accumulation of collagen, as indicated by hydroxyproline content, is attenuated in the absence of SPARC. The properties of SPARC as a matricellular protein associated with cell proliferation and matrix turnover are consistent with its participation in the development of pulmonary fibrosis.

Expression of collagens type I and IV, osteonectin and transforming growth factor beta-1 (TGFbeta1) in biliary atresia and paucity of intrahepatic bile ducts during infancy

BACKGROUND/AIMS

Biliary atresia and paucity of intrahepatic bile ducts are the main causes of neonatal cholestasis leading to hepatic fibrosis. Fibrotic evolution is slow in paucity of bile ducts as compared to the rapid progression to biliary cirrhosis in biliary atresia when cholestasis persists despite hepatoportoenterostomy. Our aim was to compare the expression of collagens type I and IV, alpha-smooth muscle actin, osteonectin and transforming growth factor beta1 in biliary atresia and paucity of bile ducts.

METHODS

Liver biopsies were obtained in 12 children with biliary atresia and in five with paucity of bile ducts. Collagens type I and IV, alpha-smooth muscle actin were detected with immunostaining. Collagens type I and IV, osteonectin and transforming growth factor beta1 mRNAs were detected by in situ hybridization.

RESULTS

Expression of mRNA and proteins was roughly parallel. In ductular proliferation areas of biliary atresia: (1) the expression of collagens type I and IV and osteonectin was increased, and was localized to periductular myofibroblasts; (2) transforming growth factor beta1 was expressed around biliary ductules, probably in inflammatory cells, and also in biliary cells. Osteonectin expression was also increased in the lobules. In paucity of bile ducts, there was no overexpression of collagens type I and IV and transforming growth factor beta1, except in the only child with marked fibrosis. However, osteonectin expression was enhanced at the periphery of the lobules, even when fibrosis was mild or absent.

CONCLUSIONS

These findings suggest that in biliary atresia ductular proliferation areas are the site of a marked production of extracellular matrix proteins in periductular myofibroblasts, probably secondary to transforming growth factor beta1 production by inflammatory cells and by biliary cells. The weak expression of transforming growth factor beta1 could explain the slow progression of fibrosis in paucity of bile ducts.

Noncollagenous bone matrix proteins, calcification, and thrombosis in carotid artery atherosclerosis

Advanced atherosclerosis is often associated with dystrophic calcification, which may contribute to plaque rupture and thrombosis. In this work, the localization and association of the noncollagenous bone matrix proteins osteonectin, osteopontin, and osteocalcin with calcification, lipoproteins, thrombus/hemorrhage (T/H), and matrix metalloproteinases (MMPs) in human carotid arteries from endarterectomy samples have been determined. According to the recent American Heart Association classification, 6 of the advanced lesions studied were type V (fibroatheroma) and 16 type VI (complicated). Osteonectin, osteocalcin, and osteopontin were identified by monoclonal antibodies IIIA(3)A(8), G12, and MPIIIB10(1) and antiserum LF-123. Apolipoprotein (apo) AI, B, and E; lipoprotein(a); fibrinogen; fibrin; fragment D/D-dimer; MMP-2 (gelatinase A); and MMP-3 (stromelysin-1) were identified with previously characterized antibodies. Calcium phosphate deposits (von Kossa's stain) were present in 82% of samples (3 type V and 15 type VI). Osteonectin was localized in endothelial cells, SMCs, and macrophages and was associated with calcium deposits in 33% of type V and 88% of type VI lesions. Osteopontin was distributed similarly to osteonectin and was associated with calcium deposits in 50% of type V and 94% of type VI lesions. Osteocalcin was localized in large calcified areas only (in 17% of type V and 38% of type VI lesions). ApoB colocalized with cholesterol crystals and calcium deposits. Lipoprotein(a) was localized in the intima, subintima, and plaque shoulder. Fibrin (T/H) colocalized with bone matrix proteins in 33% of type V and 69% of type VI lesions. MMP-3 was cytoplasmic in most cells and colocalized with calcium and fibrin deposits. MMP-2 was less often associated with calcification. The results of this study show that osteonectin, osteopontin, and osteocalcin colocalized with calcium deposits with apoB, fibrin, and MMP-3 in advanced, symptomatic carotid lesions. These data suggest that the occurrence of T/H might contribute to dystrophic arterial calcification in the progression and complications of atherosclerosis.

Primary mesenchymal cells isolated from SPARC-null mice exhibit altered morphology and rates of proliferation

SPARC (secreted protein acidic and rich in cysteine)/BM 40/osteonectin is a matricellular protein shown to function as a counteradhesive factor that induces cell rounding and as an inhibitor of cell proliferation. These activities have been defined in cell culture, in which interpretation has been complicated by the presence of endogenous SPARC. We therefore sought to determine whether cell shape and proliferation would be affected by the absence of SPARC. Mesangial cells, fibroblasts, and aortic smooth muscle cells were isolated from SPARC-null and age-matched, wild-type mice. In contrast to wild-type cells, SPARC-null mesangial cells exhibited a flat morphology and an altered actin cytoskeleton. In addition, vinculin-containing focal adhesions were distributed over the center of SPARC-null cells, whereas in wild-type cells, the number of focal adhesions was reduced, and these structures were restricted largely to the cell periphery. Although the SPARC-null fibroblasts did not display overt differences in cell morphology, the cells responded to exogenous recombinant SPARC by rounding up in a manner similar to that of wild-type fibroblasts. Thus, the expression of endogenous SPARC is not required for the response of cells to SPARC. Additionally, SPARC-null mesangial cells, fibroblasts, and smooth muscle cells proliferated faster than their respective wild-type counterparts. Null cells also showed a greater sensitivity to the inhibition of cell cycle progression by the addition of recombinant SPARC. The increased proliferation rate of SPARC-null cells appeared to be mediated, at least in part, by an increase in the cell cycle regulatory protein cyclin A. We conclude that the expression of SPARC influences the cellular architecture of mesangial cells and that SPARC plays a role in the regulation of cell cycle in mesangial cells, fibroblasts, and smooth muscle cells.

Cell shape-dependent pathway of plasminogen activator inhibitor type-1 gene expression requires cytoskeletal reorganization

Synthesis of plasminogen activator inhibitor type-1 (PAI-1), a major physiological modulator of plasmin generation, is regulated by growth factors and changes in cell shape. To evaluate the specific relationship between PAI-1 gene expression and cytoarchitecture, serum-free cultures of quiescent rat kidney (NRK) cells were exposed to cytochalasin D (CD) at concentrations that disrupt microfilament structure. Treatment with 1-10 microM CD resulted in an increased 1) incidence of rounded cells, 2) relative PAI-1 mRNA content, and 3) fraction of PAI-1 protein-expressing cells. Abrupt increases in each response were evident at a final concentration of 5 microM CD. Maximal levels of induced PAI-1 transcripts (18-fold that of control) occurred 4 hours post-CD addition and declined thereafter but remained elevated (by at least tenfold) for 24 hours. Assessment of the metabolic requirements for CD-induced PAI-1 expression by using the protein synthesis inhibitors puromycin and cycloheximide indicated that PAI-1 transcripts were regulated in a complex manner in response to CD. The predominant mode of induction reflected secondary (protein synthesis-dependent) metabolic processes, although a minor, albeit significant, primary (protein synthesis-independent) pathway was also evident. PAI-1 mRNA levels in NRK cells maintained in serum- and CD-free agarose suspension culture were low or undetectable. Relative abundance of PAI-1 transcripts in suspended cells cultured in the presence of CD, however, closely approximated that of plastic-adherent, CD-treated cells (13-fold over control). NRK cells in suspension culture with or without CD were morphologically identical, remaining spherical and unattached. It appears, therefore, that cell rounding alone is not a sufficient stimulus to induce PAI-1 expression in quiescent NRK cells and that perturbation of the actin skeleton as a consequence of CD treatment is a critical event in the inductive response. A protein tyrosine kinase is likely involved in the CD-mediated signal-transduction cascade, since induced PAI-1 expression can be down-regulated by genistein and herbimycin A but not by calphostin C or tyrphostin B46.

Regulation of human monocyte matrix metalloproteinases by SPARC

SPARC (secreted protein, acidic and rich in cysteine), also called osteonectin or BM-40, is a collagen-binding glycoprotein secreted by a variety of cells and is associated with functional responses involving tissue remodeling, cell movement and proliferation. Because SPARC and monocytes/macrophages are prevalent at sites of inflammation and remodeling in which there is connective tissue turnover, we examined the effect of SPARC on monocyte matrix metalloproteinase (MMP) production. Treatment of human peripheral blood monocytes with SPARC stimulated the production of gelatinase B (MMP-9) and interstitial collagenase (MMP-1). Experiments with synthetic peptides indicated that peptide 3.2, belonging to the alpha helical domain III of SPARC, is the major peptide mediating the MMP production by monocytes. SPARC and peptide 3.2 were also shown to induce prostaglandin synthase (PGHS)-2 as determined by Western and Northern blot analyses. The increase in PGHS-2 stimulated by SPARC or peptide 3.2 correlated with substantially elevated levels of prostaglandin E2 (PGE2) and other arachidonic acid metabolites as measured by radioimmunoassay and high performance liquid chromatography (HPLC), respectively. Moreover, the synthesis of MMP was dependent on the generation of PGE2 by PGHS-2, since indomethacin inhibited the production of these enzymes and their synthesis was restored by addition of exogenous PGE2 or dibutyryl cAMP (Bt2cAMP). These results demonstrate that SPARC might play a significant role in the modulation of connective tissue turnover due to its stimulation of PGHS-2 and the subsequent release of PGE2, a pathway that leads to the production of MMP by monocytes.

The expression of the secreted protein acidic and rich in cysteine (SPARC) is associated with the neoplastic progression of human melanoma

SPARC (secreted protein acidic and rich in cysteine) is an extracellular protein associated with tissues exhibiting high rates of cell proliferation and matrix remodeling. The current work shows that the human melanoma cell lines IIB-MEL-LES, IIB-MEL-IAN, and IIB-MEL-J and different human metastatic melanomas expressed high levels of SPARC mRNA and protein. By western blot analysis we detected a single secreted 42-kDa band in human diploid fibroblasts-conditioned medium and a 45- to 40-kDa doublet in the three melanoma cell lines and all the metastatic melanomas tested. Part of the melanoma samples and cell lines showed an additional doublet of 36-34 kDa. SPARC mRNA was expressed by the three established cell lines, 14 metastatic melanoma samples, and tumors raised in nude mice, and no spliced variants were found. The heterogeneous pattern of SPARC secreted by human melanoma cells is the result of post-translational glycosylation and a specific extracellular leupeptin-inhibitable cleavage. Unlike human fibroblasts, melanoma cells did not overexpress SPARC on addition of TGF-beta. Immunohistochemical analysis showed that SPARC was strongly expressed in 100% of primary melanomas (7 of 7) and metastatic melanomas (29 of 29), moderately expressed in most of the positive dysplastic nevi (13 of 14), and only weakly expressed in nevocellular nevi (4 of 25). Normal melanocytes did not express SPARC. The data suggest that the expression of SPARC is associated with the neoplastic progression of human melanoma.

SPARC is expressed in renal interstitial fibrosis and in renal vascular injury

Tubulointerstitial inflammation and fibrosis are critical determinants for renal function and prognosis in a variety of human nephropathies. Yet, the pathophysiology of the injury remains obscure. We investigated the expression of SPARC (secreted protein acidic and rich in cysteine) by immunohistochemistry and in situ hybridization in experimental models characterized by tubulointerstitial fibrosis and matrix expansion in rats. SPARC is a secreted glycoprotein that has been demonstrated to affect cellular interaction with matrix proteins, modulate cell proliferation, bind to and/or inhibit growth factors such as PDGF and bFGF, and regulate angiogenesis. Interstitial expression of SPARC was most prominent in passive Heyman nephritis (PHN), chronic cyclosporine A (CsA) nephropathy, and the remnant kidney model and, to a lesser extent, in angiotensin II (Ang II)-infused animals. SPARC protein and mRNA were substantially increased at sites of tubulointerstitial fibrosis/matrix expansion. In the PHN model, SPARC protein was expressed by interstitial fibroblasts that also produced alpha-smooth muscle actin ("myofibroblasts") and correlated both temporally (r = 0.97) and spatially with sites of type I collagen deposition. Interstitial cell proliferation preceded the development of interstitial fibrosis, and maximal SPARC expression (d15) coincided with the initial decline in interstitial proliferation. In the Ang II-infusion model, which is characterized by arteriolopathy and tubulointerstitial injury, an increase in SPARC protein and mRNA was also seen in injured blood vessels. SPARC was shown to be expressed by vascular smooth muscle cells and also by cells in the adventitia of hypertrophied arteries. In summary, SPARC was transiently expressed by interstitial fibroblasts at sites of tubulointerstitial injury and fibrosis, and by smooth muscle cells and cells in the adventitia of injured arteries in the Ang II-model. In addition to its proposed role in extracellular matrix deposition. the antiproliferative properties of SPARC might contribute to the resolution of interstitial fibroblast proliferation in the PHN model.

Synthesis of plasminogen activator inhibitor 1 by bovine mammary epithelial and myoepithelial cell lines

Conversion of plasminogen to plasmin provides an important source of proteolytic activity in the bovine mammary gland. Plasminogen activator inhibitor 1 (PAI-1) plays a key role in limiting plasminogen activation. The PAI-1 biosynthetic capabilities of various bovine mammary cells were determined. The immortalized epithelial cell lines MAC-T and BME-UV and the myoepithelial cell line BMM-UV were used as model systems. Northern blot analysis indicated that both epithelial and myoepithelial cells contained PAI-1 mRNA. Bovine PAI-1 was encoded by a single mRNA species approximately 3.0 kb long. BME-UV cells contained 2.0-fold (P < 0.01) the PAI-1 mRNA of MAC-T or BMM-UV cells. Reverse zymography indicated that both epithelial and myoepithelial cells synthesized PAI-1 protein with a molecular mass of approximately 50 kDa.

Inhibition of endothelial cell proliferation by SPARC is mediated through a Ca(2+)-binding EF-hand sequence

SPARC (secreted protein, acidic and rich in cysteine, also known as osteonectin and BM-40) is a metal-binding glycoprotein secreted by a variety of cultured cells and characteristic of tissues undergoing morphogenesis, remodeling, and repair. Recently it has been shown that SPARC inhibits the progression of the endothelial cell cycle in mid-G1, and that a synthetic peptide (amino acids 54-73 of secreted murine SPARC, peptide 2.1) from a cationic, disulfide-bonded region was in part responsible for the growth-suppressing activity [Funk and Sage (1991): Proc Natl Acad Sci USA 88:2648-2652]. Moreover, SPARC was shown to interact directly with bovine aortic endothelial (BAE) cells through a C-terminal EF-hand sequence comprising a high-affinity Ca(2+)-binding site of SPARC and represented by a synthetic peptide (amino acids 254-273) termed 4.2 [Yost and Sage (1993): J Biol Chem 268:25790-25796]. In this study we show that peptide 4.2 is a more potent inhibitor of DNA synthesis that acts cooperatively with peptide 2.1 to diminish the incorporation of [3H]-thymidine by both BAE and bovine capillary endothelial (BCE) cells. At concentrations of 0.019-0.26 mM peptide 4.2, thymidine incorporation by BAE cells was decreased incrementally, relative to control values, from approximately 100 to 10%. Although somewhat less responsive, BCE cells exhibited a dose-responsive decrement in thymidine incorporation, with a maximal inhibition of 55% at 0.39 mM. The inhibitory effect of peptide 4.2 was essentially independent of heparin and basic fibroblast growth factor and was blocked by anti-SPARC peptide 4.2 IgG, but not by antibodies specific for other domains of SPARC. To identify residues that were necessary for inhibition of DNA synthesis, we introduced single amino acid substitutions into synthetic peptide 4.2 and tested their activities and cell-surface binding characteristics on endothelial cells. Two peptides displayed null to diminished effects in the bioassays that were concentration-dependent: peptide 4.2 K, containing an Asp258 --> Lys substitution, and peptide 4.2 AA, in which the two disulfide-bonded Cys (positions 255 and 271) were changed to Ala residues. Peptide 4.2 K, which failed to fulfill the EF-hand consensus formula, exhibited an anomalous fluorescence emission spectrum, in comparison with the wild-type 4.2 sequence, that was indicative of a compromised affinity for Ca2(+).(ABSTRACT TRUNCATED AT 400 WORDS)

Induced expression of p52(PAI-1) in normal rat kidney cells by the microfilament-disrupting agent cytochalasin D

In established normal rat kidney (NRK) cells, synthesis of the 52 kDa type-1 inhibitor of plasminogen activator [p52(PAI-1)] is stimulated by the cell shape-modulating fungal metabolite cytochalasin D (CD). Induction paralleled the time course of morphologic change and reflected relatively specific increases in saponin-resistant p52(PAI-1) protein accumulation (approximating ten- to thirty-fold over control) and mRNA abundance (seven- to nine-fold). Augmented p52 (PAI-1) mRNA levels closely correlated with increases in 43 kDa p52(PAI-1) core protein biosynthesis. Sensitivity to tunicamycin indicated that N-linked post-translational modifications to this 43 kDa core species generated the full complement of 50 kDa (intermediate) and 52 kDa (mature) p52(PAI-1) glycosylated isoforms. CD-induced p52(PAI-1) expression occurred efficiently in quiescent NRK cells maintained under serum-free conditions as well as in fully serum-supplemented actively growing cultures. While 8-bromo-cAMP reduced both constitutive and transforming growth factor-beta-induced p52(PAI-1) synthesis by > 50%, no such inhibition was evident in short-term (4 h) CD-stimulated cultures. Long-term (24 h) exposure of NRK/CD cells to 8-bromo-cAMP did result in an approximately 34% reduction in stimulated p52(PAI-1) expression, however, levels expressed by NRK/CD+cAMP populations remained markedly elevated relative to control values. These data suggest the existence of a cell shape-dependent aspect of p52(PAI-1) expression control distinct from both the constitutive and growth factor-mediated pathways of gene regulation.

Endothelial cells exhibiting angiogenesis in vitro proliferate in response to TGF-beta 1

Transforming growth factor-beta 1 (TGF-beta 1) has been implicated in the positive regulation of angiogenesis in vivo, whereas it inhibits the proliferation of endothelial cells in vitro. To reconcile these apparently contradictory effects, we have investigated the effect of TGF-beta 1 on bovine aortic endothelial cells that exhibit spontaneous angiogenesis in vitro. We show that concentrations of TGF-beta 1 which stimulate proliferation of cells that form endothelial cords and/or tubes inhibit proliferation of the same cells grown at subconfluent densities. An increase in cell number of 35% over control cultures was achieved with 0.5 ng TGF-beta 1/ml. The proliferative effect was blocked by antibodies against TGF-beta. Immunological detection of BrdU-labeled nuclei revealed an increase greater than 220% in cells treated with TGF-beta 1. Moreover, a population of cells within the cords appeared to be a selective target for this cytokine. The stimulatory effect was not restricted to bovine aortic endothelial cells, as similar results were obtained with endothelial cells derived from rat microvessels. Significant levels of active TGF-beta 1 were detected in cultures containing cords/tubes, whereas only latent TGF-beta 1 was detected in subconfluent cultures. We show further that endothelial cells exhibiting angiogenesis in vitro secrete plasminogen activator, an enzyme that regulates activation of TGF-beta. The major increases in mRNA transcripts for extracellular matrix proteins that are typically associated with TGF-beta 1 were not seen in cells exhibiting angiogenesis in vitro. Since the formation of tubular networks requires both invasion and proliferation, we propose that TGF-beta 1 is a major morphoregulatory factor in angiogenesis that specifically controls endothelial cell proliferation and extracellular matrix turnover.

Differential effects of SPARC and cationic SPARC peptides on DNA synthesis by endothelial cells and fibroblasts

SPARC (secreted protein, acidic and rich in cysteine), also known as osteonectin, is an extracellular Ca+2-binding glycoprotein that inhibits the incorporation of [3H]-thymidine and delays the onset of S-phase in synchronized cultures of bovine aortic endothelial (BAE) cells. This effect appears not to be dependent on the functional properties of SPARC associated with changes in cell shape or inhibition of cell spreading. In this study we investigate the conditions under which cell cycle modulation occurs in different types of cells. Human umbilical vein endothelial cells, a transformed fetal BAE cell line, and bovine capillary endothelial cells exhibited a sensitivity to SPARC and a cationic peptide from a non-Ca+2-binding region of SPARC (peptide 2.1, 0.2-0.8 mM) similar to that observed in BAE cells. In contrast, human foreskin fibroblasts and fetal bovine ligament fibroblasts exhibited an increase in the incorporation of [3H]-thymidine in the presence of 25 microM-0.2 mM peptide 2.1; inhibition was observed at concentrations in excess of 0.4 mM. This biphasic modulation could be further localized to a sequence of 10 amino acids comprising the N-terminal half of peptide 2.1. A synthetic peptide from another cationic region of SPARC (peptide 2.3) increased [3H]-thymidine incorporation by BAE cells and fibroblasts in a dose-dependent manner. In endothelial cells, a stimulation of 50% was observed at a concentration of 0.01 mM; fibroblasts required approximately 100-fold more peptide 2.3 for levels of stimulation comparable to those obtained in endothelial cells. The observation that SPARC and unique SPARC peptides can differentially influence the growth of fibroblasts and endothelial cells in a concentration-dependent manner suggests that SPARC might regulate proliferation of specific cells during wound repair and remodeling.

Comparison of gene expression of extracellular matrix molecules in brain microvascular endothelial cells and astrocytes

By use of random-primed cDNA probes the expression of extracellular matrix molecules in cerebral microvascular endothelial cells (cEC) and in astrocytes from mouse brain was examined. Two phenotypically different batches of cloned cEC were used. Expression of major adhesive ECM molecules, constituting the endothelial basement membrane (i.e., fibronectin, laminin A, B and collagen IV) and of other attachment factors, such as SPARC (osteonectin), tenascin and thrombospondin 1, was examined. We have demonstrated that cEC of different morphology display variations in the expression of fibronectin (FN), thrombospondin 1 (TSP1) and collagen IV (C IV). Astrocytes were shown to contain FN, TSP1, TN and SPARC mRNA. Unexpectedly, SPARC mRNA could not be detected in any of the capillary endothelial cells examined. Therefore, we suggest that astrocytes are likely to be involved in endothelial differentiation and function in the central nervous system via ECM molecule secretion.

Cell shape changes during transition of basal keratinocytes to mature enucleate-cornified envelopes: modulation of terminal differentiation by fibronectin

Normal human keratinocytes (NHK) in submerged culture were used to assess mechanisms associated with in vitro exposure to known stimulators (sodium butyrate; NaB) and inhibitors (fibronectin; FN) of NHK maturation. A multiparameter approach was used to define cell types generated under a range of growth conditions. Differentiation induced in response to NaB occurred through a series of morphologically distinct phenotypes and culminated in the formation of enucleate-cornified envelopes. Two-dimensional electrophoresis provided a limited database to evaluate global changes in cellular protein composition as a function of induced differentiation. Proteins were identified that characterized predominantly basal cell cultures, NaB-treated cells, and fully differentiated NHKs. Growth of NHKs on FN suppressed both spontaneous and NaB-directed maturation and inhibited maximal expression of protein changes associated with the differentiated state. Thus, the composition of the extracellular matrix can modulate (at both the morphologic and protein levels) the response of basal NHKs to a potent differentiation-inducing agent. Abrogation of NHK maturation by FN was not due to adverse effects on cellular metabolism, abortive differentiation, or altered timing of induced differentiation. FN appears to exert its suppressive effect by either maintaining an early stem cell phenotype which is poorly competent for terminal maturation or attenuating an as yet unknown aspect of the NaB-initiated differentiation cascade.

SPARC antagonizes the effect of basic fibroblast growth factor on the migration of bovine aortic endothelial cells

Migration of endothelial cells is requisite to wound repair and angiogenesis. Since the glycoprotein SPARC (secreted protein, acidic and rich in cysteine) is associated with remodeling, cellular migration, and angiogenesis in vitro, we questioned whether SPARC might influence the motility of endothelial cells. In this study we show that, in the absence of serum, exogenous SPARC inhibits the migration of bovine aortic endothelial cells induced by bFGF. Similar results were obtained from two different assays, in which cell migration was measured in a Boyden chamber and in monolayer culture after an experimental wound. Without bFGF, the migration of endothelial cells was unaffected by SPARC. The inhibitory effect of SPARC on cell motility was dose-dependent, required the presence of Ca2+, was mimicked by synthetic peptides from the N- and C-terminal Ca(2+)-binding domains of the protein, and was not seen in the presence of serum. Modulation of the activities of secreted and cell-associated proteases, including plasminogen activators and metalloproteinases, appeared not to be responsible for the effects that we observed on the motility of endothelial cells. Moreover, a molecular interaction between SPARC and bFGF was not detected, and SPARC did not interfere with the binding of bFGF to high-affinity receptors on endothelial cells. Finally, in culture medium that contained serum, SPARC inhibited the incorporation of [3H]-thymidine into newly synthesized DNA, both in the absence and presence of bFGF. However, DNA synthesis was not affected by SPARC when the cells were plated on gelatin or fibronectin in serum-free medium. We propose that the combined action of a serum factor and SPARC regulates both endothelial cell proliferation and migration and coordinates these events during morphogenetic processes such as wound repair and angiogenesis.

Expression of SPARC is correlated with altered morphologies in transfected F9 embryonal carcinoma cells

SPARC (secreted protein, acidic and rich in cysteine) is a Ca(2+)-binding glycoprotein that has recently been identified as a member of a group of proteins that exert antispreading effects on various cultured cells. In addition, SPARC is induced during the later stages of F9 stem cell differentiation to parietal endoderm (PE). When treated with retinoic acid and dibutyryl cAMP, F9 cells differentiate into PE and SPARC mRNA is increased approximately 20-fold. To determine whether the chronic overexpression or inhibition of expression of SPARC would affect the morphology, attachment, or differentiation of F9 cells, we transfected undifferentiated F9 cells with cDNA encoding SPARC or anti-sense SPARC and cloned lines that expressed either elevated or reduced levels of SPARC protein. The transfected F9 cells displayed altered morphologies in culture: cells of four overexpressing lines appeared clumped and rounded, whereas those of three underexpressing lines were spread and flat, in comparison to controls. Moreover, the morphological differences persisted during differentiation of the lines to PE. The altered morphology was not due to an increased expression of collagenases and did not affect the ability of the cells to attach and adhere to tissue culture plastic. The altered phenotype of the transfected F9 cells appeared to be directly related to the level of extracellular SPARC. Since overexpression of SPARC induced rounding and aggregation of F9 cells in culture, we propose that SPARC facilitates modulation of cell-cell or cell-substrate interactions in vivo.

Immunological screening of SPARC/Osteonectin in nonmineralized tissues

SPARC/Osteonectin is a major bone-related protein that is also present in nonmineralized tissues and in platelets. As compared to bone SPARC/Osteonectin, SPARC/Osteonectin from platelets presents a slightly lower electrophoretic mobility in SDS-PAGE and a 100-fold decreased affinity for a unique monoclonal antibody, Mab2 (Malaval et al. 1991). To check the tissular diversity of SPARC/Osteonectin, protein extracts from bovine bone, nonmineralized tissues, and platelets were screened by immunoblotting and immunoradiometric assay, with Mab2 and three other monoclonal antibodies recognizing distinct epitopes. The SPARC/Osteonectin secreted by a human osteosarcoma cell line (MG63) was also tested. In all the nonmineralized tissues tested (gut, bone marrow, tendon, mesentery, artera, lens, skin, liver, and cornea), SPARC/Osteonectin presents the same immunoreactivity and electrophoretic mobility as in bone. The heavier molecular weight and Mab2-negative form present in platelets seems to be unique to this cell type. Osteosarcoma cell extracts and conditioned media give the same results as bone extracts, indicating that the low molecular weight and Mab2-positive form of SPARC/Osteonectin present in most tissues does not result from proteolytic cleavage in the matrix, but is secreted as such. Bone and platelet SPARC/Osteonectin present different patterns of sensitivity to glycosidases, suggestive of a difference in N-glycosylation. However, these treatments do not affect the decreased affinity of Mab2 for platelet SPARC/Osteonectin, which is not likely to be related to difference in N-glycosylation.